1. Field of the Invention
The present invention relates to an onboard radar apparatus.
2. Discussion of Background
For example, illustrated in FIG. 7 is known as an onboard radar apparatuses. In FIG. 7, numerical reference 1 designates a modulator; numerical reference 2 designates a voltage-controlling oscillator; numerical reference 3 designates a power divider; numerical reference 4 designates a transmitting antenna; numerical reference 5 designates an object; numerical reference 6 designates a receiving antenna; numerical reference 7 designates a mixer; and numerical reference 12 designates a signal processing device.
Operations of thus-constructed conventional apparatus will be described. The modulator 1 outputs linear voltage signals for frequency modulation (FM) composed of triangular waves. The voltage-controlling oscillator 2 generates electromagnetic waves subjected to frequency modulation by receiving these voltage signals for frequency modulation. These electromagnetic waves are divided into two portions by the power divider 3. One of the portions is inputted into the mixer 6. The other portion is outputted from the transmitting antenna 4 into space. The electromagnetic waves, i.e. transmitting electromagnetic waves, outputted into space from the transmitting antenna 4 are reflected by an object 5 and inputted in the receiving antenna 6 after a delay time Td with respect to the transmitting electromagnetic waves. In case that the object 5 has a relative velocity with respect to the radar apparatus, the receiving electromagnetic waves are inputted in the receiving antenna 6 with a Doppler shift Fd with respect to the transmitting electromagnetic waves. The electromagnetic waves received by the receiving antenna 6, i.e. receiving electromagnetic waves, are mixed with the transmitting electromagnetic waves by the mixer 7, and the mixer outputs beat signals corresponding to the delay time Td and the Doppler shift Fd. The signal processing device 12 calculates a relative range and the relative velocity with respect to the object using the beat signals.
In the next, a method of calculating the relative range and the relative velocity will be described. FIG. 8 illustrates an example of calculating the relative range and the relative velocity using the above-mentioned radar apparatus. In FIG. 8, numerical reference 100 designates a transmitting electromagnetic wave; reference B designates a frequency band width in sweeping of the transmitting electromagnetic wave; and reference Tm designates a modulation period, wherein the transmitting electromagnetic wave 100 is subjected to a frequency modulation to have the frequency band width B and the modulation Tm. Numerical references 101 and 102 designate receiving electromagnetic waves; reference R designates a range; and reference Td designates a delay time between the transmitting electromagnetic wave and the receiving electromagnetic waves, namely from reflection by the object positioned at the range R to an input in the receiving antenna.
Further, reference V designates a velocity. The receiving signal 101 relates to a case of V=0, and the receiving signal 102 relates to a case of V.gtoreq.0. Reference Fo designates a fundamental frequency. Further, when the object has a relative velocity, a receiving electromagnetic wave has a Doppler shift by Fd with respect to a transmitting electromagnetic wave. Accordingly, components of frequency included in a beat signal mixed by the mixer 7 include a frequency difference Fbu between the transmitting signal and the receiving signal in case that the frequency is increased as illustrated in FIG. 9a, and the components include a frequency difference Fbd between the transmitting signal and the receiving signal in case that the frequency is decreased as illustrated in FIG. 9b. A relative range R and a relative velocity V of the target can be obtained in accordance with Formula 1 using the above Fbu, Fbd, Tm, B, the light velocity C of 3.0.times.10.sup.8 m/s, a wavelength .lambda. of carrier wave, for example, .lambda.=5.0.times.10.sup.-3 m in case of the fundamental frequency F.sub.0 =60 GHz. ##EQU1##
In the next, provided that resolution powers, meaning minimum steps of data values discretely outputted, of the relative range R and the relative velocity V are expressed respectively by .DELTA.R and .DELTA.V, these are obtainable by Formula 2. ##EQU2##
When a plurality of objects exist, components of frequency as much as the number of the objects are contained in a beat signal when a frequency is increased and/or decreased. In this case, it is necessary to select combinations (FbuA, FbdA), (FbuB, FbdB), . . . among a plurality of components of frequency FbuA, FbuB, . . . for the increased frequency and a plurality of components of frequency FbdA, FbdB, . . . for the decreased frequency and to obtain relative ranges R and relative velocities V by Formula 1. At this time, problems occur when the combinations are erroneously selected.
For example, when two objects A, B exist, components of frequency FbuA, FbuB concerning increased frequency appear as illustrated in FIG. 10, and components of frequency FbdA, FbdB concerning decreased frequency appear as illustrated in FIG. 11. If FbdA and FbdB are erroneously combined, a relative range and a relative velocity of other than actually existing objects A, B are calculated, wherein such relative range and relative velocity are named false images.
For readiness, the references Tm, B and .lambda. are determined so that Formula 1 is transformed to be: EQU R=Fbu+Fbd (m), V=Fbu-Fbd (km/h) Formula 3
Provided that an object A having a relative range of 40 m and a relative velocity of 0 km/h and an object B having a relative range 30 m and a relative velocity of 0 km/h exist as illustrated in FIG. 12, the following results are obtained by calculating Formula 3 backward:
FbuA=15; FbdA=15; FbuB=20; and FbdB=20.
When combinations of (FbuA, FbdA) and (FbuB, FbdB) are correctly selected, the relative ranges and the relative velocities of the objects A, B can be correctly determined. However, when combinations of (FbuA, FbdB) and (FbuB, FbdA) are selected, a false image A having a relative range of 35 m and a relative velocity of 5 km/h in a leaving direction and a false image B having a relative range of 35 m and a relative velocity of -5 km/h in an approaching direction are obtained by Formula 3 as illustrated in FIG. 12.
In order to manage such a problem, for example, JP-A-4-343084 discloses a method of sampling components of frequency FbuA, FbuB concerning increased frequency and components of frequency FbdA, FbdB concerning decreased frequency by analyzing the frequencies using a fast Fourier transformation (FFT) or the like and selecting a combination having relatively small differences of signal levels utilizing an aspect that the signal levels of objects are different.
However, when the two objects A, B were detected by the above-mentioned conventional radar apparatus, in case that signal levels of the two objects A, B were coincidentally close, in other words the components of frequency FbuA, FbuB concerning the increased frequency and the components of frequency FbdA, FbdB concerning the decreased frequency had the same signal levels, it became impossible to determine combinations in reference of differences of the signal levels and false images were made by calculating relative ranges and relative velocities based on erroneous combinations of (FbuA, FbdB) and (FbuB, FbdA).